The present invention relates to a system and method of controlling an automatic transmission under a low temperature condition of a working oil in the automatic transmission.
A conventional multiple-speed automatic transmission includes a planetary gear set and a plurality of friction elements such as a clutch and a brake and performs a plurality of gear speeds by changing a combination of engagement and disengagement of the friction elements. For instance, there has been proposed an automatic transmission capable of performing a lowest gear stage which has a largest gear ratio by engaging a one-way clutch. However, if the one-way clutch is a roller one-way clutch, a vehicle could not be started even when a select lever is shifted from neutral “N” position or reverse “R” position to drive “D” position under a condition that a temperature of a working oil in the automatic transmission is low. This is because when the working oil temperature is low, an oil film formed between a roller and an inner race of the roller one-way clutch has large shear resistance to thereby prevent the roller from shearing the oil film and coming into metal-to-metal contact with the inner race. As a result, the roller one-way clutch fails to be engaged.
Japanese Patent Application First Publication No. 2006-183705 discloses an automatic transmission which contemplates avoiding occurrence of the above-described failure of starting of the vehicle at low temperatures of a working oil by engaging a brake which is disposed parallel to a roller one-way clutch.